Free cutting alloys



oFF f FREE CUTTING ALLOYS Louis W. Kempf and Walter A. Dean, Cleveland,Ohio, assignors to Aluminum Company of America, Pittsburgh, Pa., acorporation of Pennsylvania -No Drawing. Application December 28, 1935,

Serial No. 56,552

4 Claims. (01. 75147) The invention relates to aluminum base alloysDespite continual lubrication the machined surand is particularlyconcerned with aluminum base face is rough and irregular, and the chiphas a alloys containing magnesium and silicon. tendency to form acontinuous curl or spiral Aluminum base alloys containing from aboutwhich often fouls the tool or the moving parts.

5 0.1 per cent to 1.0 per cent of magnesium, and of the machine. It isimmediately apparent that from about 0.5 per cent to about 2.5 per centof there is need for .an alloy of good working charsilicon haveheretofore been used. They are acteristics and satisfactory physicalproperties, usually in the wrought condition since they are yetpossessing such favorable machining properradily susceptible tomechanical deformation ties that the complete machining operations maysuch as rolling, forging, or extrusion. It is posbe performedeconomically and successfully, and slble, by appropriate thermaltreatments to very y be du ti e f apl s n su f appearsubstantially'improve their physical properties. ance. After ahigh temperature s l titreatment; they Accordingly an object of our invention is the do notage-harden to any great extent at room Provis on of n um m base al yContaining 16 temperature but may be artificially aged by temfrom about0.1 to about 1.5 per cent of magperatures somewhat in excess of roomtemperanesium and from about 0.5 to 2.5 per cent of ture as is wellknown in the art. For this reasilicon, which may be readily andeconomically son these alloys may be wrought to substantialmachined- 1yfi al form, th subjected t high tempera,- Our invention resides in thediscovery that 20 ture solution treatment, and while in a relativeheforegoing Obj s e f cted by adding at ly soft condition they may begiven certain final least One O t e elements lead and tin to theWorking, steps at low temperature. Since th y aluminum-magnesium-siliconalloy. When these finishing steps may be performed t any fied below, thealloys are known as free cutting venient time. They may ultimately beartificialfree ac i alloys because y a be ly aged if the maximum tensilestrength, yield machined 'e p yo a Similar alloys Withstrength, andhardness are d d, out the lead or tin, and yet have as good or a Sinceall commercial l mi ba alloys better finished surface. After an extendedseit is to be understoodthat the amount of silicon alloying elementswhich h a favorable eflec discussed herein, and hereinafter claimed,repupon the machining P p s 0 aluminumresents the total quantity ofsilicon present and magnesium-Silicon yS- n O fi O t s that the siliconcontent of the aluminum used effect We term lead a t free ma n n shouldbe known in order to provide a basis for elements- We have furtherdiscovered a the determining th e tu composition of t simultaneouspresence of both of these elements aluminum-magnesium-silicon ,11 isproductive of an improvement in free machin- There are, however, someapplicatjons whereing characteristics which is considerably greater inalum num-magnesium-silicon alloys a h r than that caused by the presenceof the same 40 inabove disclosed might be conveniently and total amountof either one alone. For example, profitably used except for an inherentdisadthe addition p cent of d a d D vantage which militates againsttheir use in cent of t to an aluminum base alloy containing theproduction of certain articles requiring exactabout D cent of magnesiumd De ing machining operations, Mechanical cutting cent of siliconeifects a greater improvement in r tions such asdrilling, shaping, or 1,1; tmachining quality than does the addition of 1.0

ting are successfully carried out only by using P cent f either l tin sycertain precautions which increase the cost of Since these two elementshave Such a production and perhaps favor th choice of vorable effect onthe machining characteristics of other metal or alloy which can bemachined 'elll inum nagnesium-silicon alloys, it is formore readily butwhich is not s d i b i tunate that they are also of relatively lowmeltother respects, as for example, in physical propg po nt, a a Whichmakes p b e e r ad- .erties. When alloys are difiicult to machine thisditien in molten aluminum in the p a disadvantagev becomes evident, inmany cases, without the intervention of so-called rich althrough rapidwear of the cutting tool edge, loys. It is also a fact that the elementslead so that frequent tool re-sharpening is required. and tin form withaluminum a series of alloys 2 of limited liquid solubility. Within therange disclosed and claimed howeverthese free machining elements may beadded without unusual difificulty. We suspect that this characteristic 5feature of the elements lead and tin may be one of the significantfactors which contribute to their free machining efiect. We believe thatthis effect is further-strengthened by distributing the free machiningconstituent relatively homogeneously throughout the solid matrix, sincelead and tin are also practically insoluble in the solidaluminum-magnesium-silicon base alloy.

The lead or tin, if used alone, may be added in amounts between about0.05 per cent and 6.0 per cent. If used together the total amount offree machining constituent should not be less than 0.05 per cent, andneed not exceed 6.0 per cent. We have determined that the maximum limitof, 6.0 per cent of free machining metalis suflicient for satisfactorycommercial results,

since although the free machining effect persists beyond this amount,certain of the other physical properties may be unfavorably affected.

Aluminum-magnesium-silicon alloys containing one or both of the freemachining elements lead or tin may be machined more rapidly, with lesstool wear, less tool sharpening, better quality of chip and bettermachined surface than the same base alloys without the free machiningadditions and as previously indicated the simultaneous presence of tinand lead is considerably more helpful to the machining quality of thealloy than an equivalent total amount of either lead or tin alone. 7,

As a preferred alloy within the range of the respective elementsdisclosed hereinabove we su gest an alloy containing about 0.6 per centof magnesium, 1.0 per cent of silicon and a total of 3 per cent of leadand tin, the balance being 40 aluminum. For some applications which donot require a high degree of free machining quality a lesser amount ofthe free machining metal may be added to the alloy base, for instance0.5

per cent of lead or 0.5 per cent of tin, 45 per cent total of lead andtin.

For certain purposes, notably the "improvement of tensile strength,hardness and grain structure, the alloys as hereinabove disclosed may beimproved by the addition of one or more 50 of the group of elementscomposed of molybdenum, vanadium, titanium, tungsten, zirconium andchromium. From 0.05 to 1 per cent of any one of these elements may beused alone, but it more than one is employed the total amount 55 shouldnot exceed about 2 per cent.

' March 15, 1934.

-a critical period of time.

. to 2.5 per cent silicon,

The free machining alloys which have been described hereinabove maybesubjected to the thermal treatments well known in the art to improvetheir strength and hardness. We have found that a solution heattreatment and subsequent aging does not impair the free machiningquality of the alloys and in many instances the treatment even tends toimprove this property. For many purposes a relatively high strength andhardness are necessary to the successful performance of the machinedarticle and hence the alloy must be heat treated. This treatment isgenerally applied prior to the machining operation.

As hereinabove indicated the free machining elements, by reason oftheir'low melting point, may be added to the molten aluminum in puremetallic form. However, since some difliculty may be encountered inintroducing them in the higher percentages of scribed in No. 1.959.029,issued Briefly it involves heating the melt to a somewhat highertemperature than is customary, and vigorously stirring it in excess ofThe term aluminum as used herein and in the appended claims embraces theusual impurities found in aluminum ingot of commercial grade-or pickedup in the course of the ordinary handling operations incident to meltingpractice.

We claim:

our disclosed range we prefer to use the method which is more fullyde- 1. A free cutting aluminum base alloy containing from 0.1 to 1.5 percent magnesium, from 0.5 and a total of from 0.05 to 6 per cent of oneof the group of free machining elements composed of lead and tin, thebalance being substantially aluminum.

2. An aluminum base alloy containing from 0.1 to 1.5 per cent magnesium,from 0.5 to 2.5 per cent silicon, and from 0.05 to 6 per cent of lead,the balance being aluminum. v

3. An aluminum base alloy containing from 0.1 to 1.5 per cent magnesium,from 0.5 to 2.5 percent silicon, and from 0.5 to 6 per cent of tin, thebalance being aluminum.

4. An aluminum base alloy containing from 0.1 to 1.5 per centmagnesium,from 0.5 to 2.5 per cent silicon, from 0.05 to 6 per cent of one of thegroup of free machining elements composed of lead and tin, and from:0.05 to 2 per cent of hardening metal from the group composed ofmolybdenum, vanadium, titanium, tungsten, zirconium and chromium, thebalance being aluminum.

- LOUIS W. KEMPF.

WALTER A. DEAN. v

